In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, transmission schemes and reception schemes based on the use of narrow beams might be needed at high frequencies to compensate for propagation losses. For a given communication link, a beam can be applied at both the network side (such as at the transmission and reception point (TRP) of a network node) and the user side (such as at terminal devices served by the network node). A beam pair link (BPL) is defined by the beam used by the TRP (denoted TRP beam) for communicating with the terminal device and the beam used by the terminal device (denoted UE beam) for communicating with the TRP. Each of the TRP beam and the UE beam could be used for any of transmission and reception. Likewise, there could be separate BPLs for downlink communications (where the TRP beam is a transmission (TX) beam and where the UE beam is a reception (RX) beam) and uplink communications (where the TRP beam is an RX beam and where the UE beam is a TX beam).
In general terms, a beam management procedure is used to discover and maintain BPLs. A BPL is expected to be discovered and monitored by the network using measurements on downlink reference signals used for beam management, such as channel state information reference signals (CSI-RS). The CSI-RS for beam management can be transmitted periodically, semi-persistently or aperiodic (such as being event triggered) and they can be either shared between multiple terminal devices or be device-specific.
In order to find a suitable TRP beam the TRP transmits CSI-RS in different TRP TX beams on which the terminal devices performs reference signal received power (RSRP) measurements and reports back the N best TRP TX beams (where the value of N can be configured by the network). Furthermore, the CSI-RS transmission on a given TRP TX beam can be repeated to allow the terminal device to evaluate suitable UE beams, thus enabling so-called UE RX beam training.
In further detail, during BPL establishment (e.g. using the example of UE RX beam training for a terminal device with an analog antenna array), it is expected that the terminal device scans through a set of UE RX beams, which typically are narrow pencil beams pointing in different directions, and then selects the UE RX beam out of the set of UE RX beams that gives the highest measured reference signal received power (RSRP). The reason for using narrow beams is that the narrower the beams, the higher the antenna gain. Such narrow high gain beams are especially useful in line of sight channels where the channel angular spread seen by the terminal device is rather small.
It is foreseen that the terminal device could be configured to generate a plurality of different such narrow beams. Evaluating all possible beams during the beam training could therefore be time consuming as well as requiring large amounts of network resources (since one occurrence of the reference signal used must be transmitted from the TRP for each UE RX beam). Therefore a selection has to be made regarding which beams to include in the beam training. Such a selection can result in that the best beams are not used during the beam training. Selecting the best UE RX beam out of the set of UE RX beams evaluated during a UE RX beam training procedure might thus result in the best UE RX beam not being found. The same situation could occur during TRP RX beam training, during UE TX beam training or during TRP TX beam training.
However, there is still a need for improved beam training for radio transceiver devices in a communications network.